Telomerase is a ribonucleoprotein enzyme that uses its internal RNA component as a template for the synthesis of DNA on the ends of chromosomes during cell replication. In mammals, telomerase is normally found only in embryonic and germ cells and in low levels in renewal tissue such as leukocytes. Most somatic cells have no telomerase. In contrast telomerase is found in high levels in malignant cells, allowing these cells to divide indefinitely, thus immortalizing the cell clone. Telomerase has been detected in almost 90 percent of tumors of all types tested to date. All measurements of telomerase activity in tumors reported to date have used cellular material. To our knowledge, no previous attempts at the measurement of telomerase activity in non-cellular fluids, such as plasma, have been made. We have developed preliminary data showing that telomerase is detectable in the plasma of patients with active lung cancer. Further, we have obtained preliminary data showing, in a small number of cases, a correlation between the plasma telomerase levels and clinical stage of disease. Based on these observations, we propose to examine a larger group of patients with a variety of cancers, including malignacies of the lung, esophogus, and gastrointestinal tract. We will evaluate the utility of plasma telomerase measurements as aids in the diagnosis of malignancy and as a monitoring tool during therapy to predict relapse.